Apparatus for measuring potentials



May 21, 1940. s. B. YOUNG APPARATUS FOR MEASURING POTENTIALS Filed April 6, 1938 INVENTOR. j'aul filmy WWW ATTORNEYS Patented May 21, 1940 UNITED STATES PATENT OFFICE APPARATUS FOR mizasuamc POTENTIALS Saul B. Young, Drum. N. Y.

Application April 6, 1938, Serial No. 200,309

. 4 Claim.

This invention relates to electrical measuring apparatus and while not limited thereto has particular reference to an improved apparatus for measuring minute potentials in circuits which include a solution.

The invention has particular application to the measurement of hydrogen-ion concentration of solutions in terms of pH units.

The invention comprehends in an apparatus of the character described, means for compensating for or balancing out of juncture potentials in the circuit, together with means for adjusting the resistance of the circuit to correspond with the temperature of the solution under test and means for balancing the potential across a high resistance medium permeable to hydrogen-ions and which medium surrounds the solution to be tested.

The-invention more particularly comprehends in an apparatus of the character described, a

two stage direct current voltage amplifier circuit which includes a null point visual indicator and means for varying the potential in said circuit so as to fully illuminate the target of the visual indicator, said amplifier circuit being adapted to be thrown in circuit with a high resistance medium permeable to hydrogen-ions and which medium surrounds thesolution to be tested for acidity, a balancing temperature resistance for compensating for the temperature of said solution, a buffer potentiometer for eliminating or balancing out juncture potentials in the circuit with said high resistance medium, a balancing potentiometer graduated in pH units and a standard cell adapted to be inserted in circuit with the potential thereof opposed to the potential of the balancing graduated potentiometer so as to adjust the variable resistance of the balancing potentiometer circuit in order to obtain a definite potential drop across the balancing graduated potentiometer.

The invention further embodies an indicator of the said character in the amplifier circuit which functions to visually indicate when the potential of one portion of the series circuit is equal and opposed to the potential of the other portion of the series circuit.

Vifith the foregoing and other objects in view, reference is now made to the following specification and accompanying drawing in which is illustrated a preferred form of the invention, while the appended claims cover variations and modifications thereof which fall within the spirit of the invention.

In the drawing:

Fig. 1 is a diagrammatic view of the apparatus illustrating the glass electrode in perspective with portions thereof in section.

Fig. 2 is a sectional view through the stop cock of the glass electrode taken on the line 2-2 of Fig. 1 and illustrating in full and dotted lines the several positions of one of the passageways of the rotating core thereof.

Fig. 3 is a similar view taken on line 3-3 of Fig. 1 and illustrating in full and dotted lines the positions of the other passageway 01 the rotating core of the stopcock.

Referring to the drawing by characters of ref-, erence, the apparatus includes a glass electrode 'H, a balancing calibrated potentiometer l2, a

standard cell IS, an uncalibrated buffer potentiometer I4, a temperature resistance l5 calibrated in thermal units and a two stage voltage amplifler circuit Hi.

The glass electrode ll preferably consists of a glass jacket I I having an upwardly extending offset tubular extension l8 and a glass inner tube or membrane l9 protruding through and sealed to the opposite ends thereof. The upper end of the inner tube or membrane 19 is connected to a funnel 20 while the lower protruding end thereof is connected to a five way stop cock 2| by a rubber ferrule 22.. A tube 23 is afilxed to the casing of the stop cock 2| in communication with one of the ports thereof and through said port is adapted to communicate with a passageway 24 in the rotating core 25 thereof. The tube 23 has mounted thereon an enlarged bulb 26 providing a liquid reservoir with a filling opening 21 in the top thereof.

The rotating core 25 of the stop cock is also provided with a second passageway 28 therethrough which is adapted to be placed in communication through a port in the casing of the stop cock with one leg 29 of a bifurcated conduit 30 connected to and communicating with an upwardly extending vessel ill with the open end of the conduit 30 extending into and terminating adjacent the upper end thereof. The other end of the passageway 28 is adapted to communicate through a port in the casing of the stop cock with a discharge tube 32. The other end of the passageway 24 is adapted to be placed in communication through a port in the casing of the stop cock with the opposite leg 33 of the bifurcated 5o conduit 30. The vessel M is provided with a tube 34 connected therewith and extending upwardly from the lower end thereof, the juncture of the said tube with the vessel 3| being sealed to prevent flow of liquid therebetween. A platinum wire 66 is embedded in the sealed connection between the said-tube and vessel 6| with the opposite ends of the wire 66 projecting respectively into the said tube and vessel.

The tube 36 is filled to a point adjacent the upper end thereof with mercury indicated by the reference character 66 and in which the project: ing end of the platinum wire 36 is immersed.

' The mercury 66 constitutes one of the terminals of the glass electrode II and extending into the upper open end of the tube 36 and immersed therein is a conductor 61 leading from the glass electrode. The vessel BI is also provided with mercury indicated by the reference character 38 suillcient in quantity to cover and immerse the opposite end of the platinum wire and superimposed upon the mercury therein is a paste indicated by the reference character 69, composed of mercurous chloride mixed with mercury and potassium chloride crystals.

The tubular extension I6 of the glass jacket I! has fitted in the open end thereof a glass tube 6I sealed at its lower end and having embedded therein a platinum wire 62 which has its free end projecting downwardly into the tubular extension I6 and its upper end immersed in mercury 66 in the said tube. The mercury 66 forms the opposite terminal of the glass electrode II and in which is immersed the end of a conductor 66. The lower protruding end of the platinum wire 62 is silver plated and chloridized.

The conductor 66 is connected at its opposite end to the on terminal 66 of a single pole, double throw switch 66, the central terminal 61 of which has connected thereto a conductor 66 constituting one of the conductors leading to the two stage amplifier circuit I6. The opposite or "zero" terminal 66 of the said switch has connected thereto a conductor 66 leading to and connected withone of the middle poles 6i of a double pole, double throw switch 62, the opposite middle pole 66 thereof hasconnected thereto a conductor 66 which leads to and is'connected with the terminal 66 thereof and from which leads a conductor 66 forming the opposite lead to the two stage amplifier circuit I6. The terminal 61 opposite the terminal 66 has a conductor 66 leading to the negative side of the standard cell I6 and from the positive side of which cell a conductor 66 leads to a middle terminal 66 of a triple pole double throw switch 6|.

The pole 62 of the switch 62 on the same side thereof as the terminal 61 has a conductor 66 leading therefrom to the circuit of the balancing calibrated potentiometer I2. The remaining terminal 66 of the switch 62 located on the same side as the terminal 66 thereof has a conductor 66 leading to one of the middle poles 61 of a double pole reversing switch 66. The outside middle pole 66 of the switch 6i has connected thereto the conductor 61 of the glass electrode II. The outside terminals II and 12 have conductor wires I6, 16 and 16 leading therefrom respectively to the middle pole I6 of the reversing switch 66, the contact ll of the calibrated resistance 16 of the potentiometer I2 and-the movable contact arm 16 of the bufier potentiometer I6. The central pole 66 of the switch 6| has a conductor 6I leading therefrom and connected to the end contact 62 of the calibrated resistance I6 of the potentiometer I2. The central terminal 66 of the switch 6| has a conductor 66 leading therefrom and connected to the movable contact arm 66 of the temperature resistance I6, the central tera,ao1,sso

minal as also being connected to the nee terminalssll of the switch 6i by a conductor".

The balancing calibrated potentiometer i2 disposed between theconductors 66 and 6I' includes the aforesaid calibrated resistance "I6 preferably consisting of fourteen contacts separated by ohm resistance steps and with'a conductor 66 connecting the 'zero" contact 86 thereof. with the terminal 66 of a continuous\ wire wound resistance 6i of 660 ohms. calibrated and designated by indicia ranging from "zero to "thirteen respectively The continuous wire wound resistance Si is provided with graduations from "zero" to "one hundred" for providing fractional readings between each of the unit readings of the calibrated resistance I6.

'The opposite terminal 62 of the resistance 6i is connected by a conductor 66 to a resistance 96 of 18.1 ohms which resistance is connected on its The said contacts are 10 opposite side to the conductor 66. The calibrated resistance 18 and the continuous wire wound resistance M are provided with movable contact arms and 66 respectively which arms are connected by conductors 61 and 66 to the terminals 66 and I66 of. the reversing switch 66 and from the terminals 66 and I66 to the diagonally opposite terminals respectively "I and I62. The continuous wire wound resistance 6i has a con- ,ductor I66 shunted across the terminals 66 and 62 thereof and with a resistance I66 of ohms in said shunt circuit in parallel with the 600 ohm resistance thereof.

The balancing calibrated potentiometer I2 also includes a conductor I66 extending from the resistance 66 to the negative side of a 1 volt battery I66 and from the positive side thereof to a movable arm I61 of a 1000 ohm resistance I66, a

conductor I66 connecting the said resistance "6' to the movable arm 66 of the temperature resistance I6. The temperature resistance I6, graduated in thermal units, is of 200 ohms and is connected to the terminal 62 of the calibrated resistance I6 through a 200 ohm resistance 6.

The buffer potentiometer I6 includes a 400 ohm resistance III connected on one side by a conductor II2 to the negative side of a 1 volt cell II6, the positive side of which is connected by a conductor I I6 to a 2000 ohm resistance II6, the opposite side of said resistance being connected to the opposite side of the resistance III and to the conductor 16 at III.

The two stage voltage amplifier circuit I6 includes a 606 amplifier tube, designated by the reference character I 26, and a 6E5 amplifier tube I2I. The 6C6 tube is a triple grid tube of the heater cathode type and includes a suppresser grid I22, a control grid I26, a screen grid I26, a plate or anode I26, a cathode I26 and a filament I21 which filament is connected to the ter-.

minals of a 6 volt storage battery I26 as at I26 and I66 and with a filament switch I6I interposed in circuit therewith. This tube is constructed to operate on a free grid potential of approximately minus 1% volt which is obtained by putting the cathode at a potential of approximately plus 1 volt. The free grid potential of the control grid is obtained with the terminal 61 of the switch 66 positioned in open relation or out of contact with either of the terminals 66 and 66 thereof, the grid being free the tube will assume a free potential between the control grid I26 and the cathode I26 of 1% volts.

The 6165 tube, designated by the reference character III. includes a control grid I66, a plate or anode I66, a cathode I66, a filament I66, the

The cathode I20 is connected to an adjustable terminals of which are likewise in circuit with the opposite sides of the 6 volt battery I28 and connected to the terminals I29 and I30 and having the filament switch I3I interposed in circuit therewith. The 6E5 tube is a high vacuum heater, cathode type tube which indicates visually, by means of a fluorescent electron target I31 the effects of a change in the controlling voltage applied to the control grid I33. The flow of electrons from the cathode I35 to the plate or anode I34 is by means of the conductor I38 which is ,connected to the negative side I39 of a 90 volt battery I40 and from the positiveside I 01' said battery through the conductor I42 to a fixed resistance I43 of 500,000 ohms to the anode terminal of the tube. The flow of current is to be considered as emanating from the positive terminal I4I of the battery 40 through the conductor I42 thence through the 500,000 ohm resistance I43, thence plate or anode I34, thence across the gap in the vacuum tube to the cathode I35 which is directly connected to the conductor I38 which leads to thenegative side I39 of the said battery. However, it is to be understood that the flow of electrons is in the opposite direction.

The control grid I33 of the 6E5 tube designatedby the reference character I2I is connected to the negative terminal of a 22% volt dry battery I44 by means of a conductor I45, the positive terminal of said battery being connected by a conductor I40 to a juncture point I41. The plate I25 of the 6C6 tube, designated generally by the reference character I20 is also connected to the juncture point I41 by a conductor I48. The plate I25 having approximately a voltage of plus 19.2 volts will produce a voltage at the juncture I41 of the same amount. The plate I25 of the 6C6 tube has a voltage 5f 19.2 volts for the reason that the same is connected by means of the conductor I48 and a conductor I49 to one end of a 5 million ohm resistance I50, the opposite end of which resistance is connected by a conductor I5I to the 45 volt terminal I52 of the 90 volt battery. The negative terminal I39 of said 90 volt battery I40 is connected to the ground as at I53. The current flowing in the said circuit being 5.16 micro-amperes and the potential drop across the 5 million ohm resistance I50 being 25.8 volts, the voltage at the juncture I41 is plus 19.2 volts.

The suppresser grid I22 of the 606 tube is connected by the conductor I55 to the cathode I26. This arrangement places the suppresser grid under the same potential as the cathode. screen grid I24 is connected by a conductor I56 to an adjustable terminal I51 of a 20,000 ohm resistance I58 and which resistance on one side is connected to the negative terminal I39, through a 20,000 ohm resistance I 59, of the 90 volt battery I40 and on the opposite side to the 22 volt terminal I60 thereof with a 10,000 ohm resistance I6] and switch I 62 between the 22 volt terminal thereof and the resistance I58. By this arrangement potential variance between 9 and 18 volts may be applied to the screen grid I24 by moving the adjustable terminal I51 on the resistance I58. This is done to adjust the illumination on the electron target I31 with the control grid I23 free as hereinbefore described. With the control grid I33 at approximately minus 3.3 volts, the electron target I31 is completely illuminated and in the operationof the apparatus the terminal I51 is adjusted on the resistance I58 so as to maintain the control grid I33 at approximately minus 3.3 volts so as to effect the proper illumination of the electron target I31 with the said control grid I23 free as hereinbefore described.

The

terminal I03 of a 6000 ohm resistance I64, the

opposite ends of which resistance are connected respectively to the 6 volt battery I 28through a 15,000 ohm resistance I05o'n the one side and 50,000 ohm resistance I on the other side and with the switch I3I interposed between the resistance I60 and the positive side of the battery I28. The negative side of the 6 volt battery I28 is connected to the ground as at I53 as is also the conductor 56 from the terminal 55 of the switch 52. voltage amplifying circuit I6 is connected as hereinbefore described to the free terminal 41 of the switch 46 and with its opposite ends connected to the control grid I23.

When the switch 46 is thrown to connect the free terminal 41 to the zero terminal 49 thereof, and the switch 52 thrown to connect the terminals I and 55, the control grid I23 of the 606 tube is connected to the ground through the conductor 50 and from the terminal 5| of the switch 52 to the terminal 55 thereof and thence through the conductor 56 to the ground I53. With the control grid I23 thus connected to the ground, the adjustable terminal I63 of the resistance I64 is adjusted until the control grid I33 is brought to a potential of minus 3.3 volts which adjustment places the cathode I20 at-plus 1 volts with reference to the control grid I23. Adjusting the terminal I63 of the resistance I64 in this manner, the electron target I31 is brought to complete illumination and the voltage at the juncture I41 is approximately 19.2 volts.

In using the apparatus for determining the acidity of an unknown solution in terms of pH units, the same is operated in the following manner:

With the switch 46 in open relation, the switches I3I and I62 are closed thus placing the control grid I23 of the 606 tube at a free grid potential of approximately 1 volts. The terminal I51 is then adjusted on the resistance I58 to'thereby adjust the voltage applied to the screen grid I24 so as to place the control grid I33 of the 6E5 tube at approximately minus 3.3 volts at which voltage the electron target I31 is completely illuminated. The switch 46 is then thrown to zero position and the switch 52 thrown so as to connect the terminals 5I and 55, the conductor 48 is then connected through the switch 46 to the conductor 50 and from the conductor 50 through the switch 52 to the conductor 56, thus closing the amplifier circuit I6 and connecting the control grid I23 to ground at I53. The terminal I63 is then adjusted on the re- The opposite conductor 48 of the sistance I64 until the control grid I33 is brought to the potential of approximately minus 3.3 volts which adjustment places the cathode I20 of the 6C6 tube at plus 1 volts with reference to the potential of the control grid I23 and producing a voltage at the juncture I41 of approximately 19.2 volts and completely illuminating the electron target I31.

The switch 6I is then thrown to connect the terminals with 06 and 69 with 12 and the switch 52 thrown to connect the terminal 5| with the terminal 52, the switch 46 being retained in the previously set position connecting the terminal 41 with the terminal 49 thus placing the standard cell in circuit with the potentiometer I2 and with the voltage amplifier circuit I6. This is done to balance the potentiometer I2 against the standard cell I3 which is efiected by moving the adjustable terminal I01 sistance II is set to the position which com sponds with the temperature oi the glass electrode II and the solutions to be employed therewith the switch 2| retained in the previously set position, the switch 22 is then thrown to connect the terminals ll with-ll and 22 with 65. The glass electrode II is then supplied with a standard solution or a solution of known acidity and the movable contact arms II and 22 set I to the known pH of the said standard solution and the switch 42 thrown to the "on" position connecting the terminal 25 with 21. Thus the standard cell I2 is out of circuit and the glass electrode is interposed in circuit through the condu'ctor 42, switch 42 and conductor 22 with the voltage amplifier circuit it and through the conductor ll, switch 22 and reversing switch 22 in the direct position to the movable contact arm 85 of the potentiometer II. The opposite conductor 21 of the glass electrode II is connected through the switch ii to the adjustable terminal I! of the buffer potentiometer I2 and from the potentiometer ll through the conductor 12 to the terminal 16 of the double throw switch 68 and thence to the terminal 22 thereof and movable contact arm 25 of the potentiometer l2. The movable contact arm 12 of the buffer potentiometer II is then adjusted on the resistance so as to again bring the electron target |2 1 to complete illumination. Adjusting the butter potentiometer H in this manner with a solution of known acidity in the glass electrode balances out the juncture potentials in the circuits, so that when the potential across the inner tube or membrane I2 is obtained for an unknown solution by the readings on the calibrated resistances l2 and SI of the calibrated potentiometer l2, the same will be the true readings of the potential due to the hydrogen-ion concentration in the unknown solution. The standard or known solution is then removed from the glass electrode I, the electrode supplied with the unknown solution and the movable contact arms 28 and 22 adjusted so that the electron target I21 is fully illuminated whereupon the unit reading on the calibrated resistance l2 and the fractional reading on the resistance II will give the acidity of the unknown solution in terms of pH units. e

In employing the glass electrode ii in the apparatus, the glass jacket I! is filled with a normal solution of hydrochloric acid so that the silver plated and chloridized'end of the platinum wire 22 is immersed therein. The rotating core 25 of the stopcock 2| is set so that the inner tube or membrane I2 is connected through the passageway 22 to the discharge tube 22. The inner tube or membrane i2 is then filled through the funnel 22 with the said standard solution. as aforesaid. the passageway 22 being allowed to remain open until some of the'solution is discharged through the discharge tube 22 after which the rotating core 22 is rotated in a clockwise direction closing the outlet 22 so that the solution is retained in the inner tube or membrane I! and the outlet 22 is in communication with the passageway 22 to the leg 22 of the bifurcated conduit 22. This connects the bifurcated conduit 22 through the leg 22 thereo! and the passageway 24 in the rotating core to the tube 22 having mounted thereon the enlarged bulb 22.;' me bulb 22' is supplied through the opening 21 thereof with potassium chloride solution which iiows through the passageway 22, the bifurcated conduit 22. passageway 22 and discharge tube 22 and upwardly through the open end of the conduit 22 into the vessel 2| and in contact with the paste 22 in the bottom thereof.

The vessel 2| is filled with the potassium chloride solution to a point above the upper end of the tube 22 after which the rotating core 22 is again rotated in a clockwise direction thus closing the discharge tube 22 and the passageway 24 to the tube 22 and bulb 22 and connecting the leg 22 with the passageway 22 and in communication through the ferrule 22 with the inner tube or membrane l2 so that the potassium chloride solution in the vesselll, tube" and leg 22 is broughtsinto abutting relation in the passageway 22 of the stop cock 2|, with the standard solution in the inner tube or membrane II. In this manner the conductor, 21 is in circuit through the mercury 22 in the tube 24, the platinum wire 22, mercury and solutions in the vessel 2| and conduit 22 with the standard solution in the inner tube or membrane II. The opposite conductor 24 is in circuit through the platinum wire 42 with the hydrochloric acid in the glass jacket II and with the inner tube or membrane II which completes the circuit with the standard solution contained in the inner tube or membrane I2, the said inner tube or membrane being a high resistance medium permeable to hydrogen-ions in the said solution.

When the butter potentiometer It has been adjusted as herelnbetore described with the glass electrode supplied with a standard solution whose acidity is known, the said solution is then drained from the glass electrode and the unknown or solution whose pH is to be determined is then inserted in the inner tube or membrane l2 and the rotating core 22 of the stop cock 2| adjusted as hereinbeiore described to again an the legs 22 and 22 of the conduit 20 with fresh potassium chloride solution from the bulb 22. This will permit of the potassium chloride solution in the leg 22 flowing into abutting relation in the passageway 22 in the stop-cock 2| with the unknown solution in the inner tube or membrane II as hereinbeforedescribed after which the movable contact arms 22 and 92 are adjusted until the electron target I21 is illuminated, the readings on the calibrated resistances 12 and II for the setting of the contact arms 25 and 12 indicating the acidity of the unknown solution in terms of pH units and fractions thereof.

The apparatus may also be employed for indicating electromotive force. In the event that a solution is to be tested for electromotive force, the glass electrode is employed as hereinbetore described. while if a metal or other material is to be tested the conductors 21 and 24 are connected to the opposite ends thereof.

In operating the apparatus for the determina-,

is then adjusted until the electron target I31 is completely illuminated which indicates that the potential of the balancing potentiometer between the points 11 and I6! is equal and opposed to that of the standard cell. The switch 52 is then thrown to connect the terminal with 55 and 53 with 65 thus disconnecting the standard cell from the circuit and placing the balancing potentiometer l2 in circuit through the conductors 3'! and M with the material to be tested. The switch 46 is then thrown to connect the terminal 41 with 45 and thereafter the movable contact arms 85 and 96 are adjusted on the resistances l8 and 9| until the electron target is again fully illuminated, the reading on the resistances 18 and SI giving the electromotive force of the material being tested. For this determination the resistance 18 of the balancing potentiometer I2 is graduated in units of electromotive force and the resistance between adjacent contacts thereof each being of one hundred millivolt steps. If in balancing the potentiometer l2 against the material being tested, it is found that the potential thereof is not opposed to the potential of the said material, the reversing switch 68 is thrown to the opposite position so as to place the'potential of the balancing potentiometer l2 opposed to that of the material being tested.

What is claimed is:

1. In an apparatus for indicating potentials, a high resistance medium permeable to hydrogenions and adapted to be interposed in contact with a solution to be tested for the potential thereof, a balancing potentiometer for indicating the potential of said solution due to the hydrogen-ion concentration thereof, a direct current voltage amplifier circuit including a visual indicator and means in said circuit for varying the potential thereof to effect the complete illumination of the said indicator, means for connecting said amplifier circuit and said balancing potentiometer in series with said high resistance medium, a buffer potentiometer for compensating for juncture potentials in said series circuit, means for connecting said buffer potentiometer in said series circuit, a variable resistance in circuit with said high resistance medium for compensating for the temperature of the said solution, a standard voltage cell, and means for connecting said cell in circuit with the balancing potentiometer for balancing the potentiometer against the standard cell so as to obtain a definite potential drop across the balancing potentiometer.

2. In an apparatus for indicating potentials, a high resistance medium permeable to hydrogenions and adapted to be interposed in contact with a solution to be tested for the potential thereof, a balancing potentiometer for indicating the potential of said solution due to the hydrogen-ion concentration thereof, a direct current voltage amplifier circuit including a visual indicator and means in said circuit for varying the potential thereof to eflect the complete illumination of the said indicator, means for connecting said amplifler circuit and said balancing potentiometer in series with said high resistance medium, a buffer potentiometer for compensating for juncture potentials in said series circuit, means for connecting said buii'er potentiometer in said series circuit, a standard voltage cell, and means for connecting said cell in circuit with the balancing potentiometer for balancing the potentiometer against the standard cell so as to obtain a definite potential drop across the balancing potentiometer.

3. In an apparatus for indicating potentials, a high resistance medium permeable to hydrogenions and adapted to be interposed in contact with a solution to be tested for the potential thereof, a balancing potentiometer for indicating the potential of said solution due to the hydrogen-ion concentration thereof, a direct current voltage amplifier circuit including a visual indicator and means in said circuit for varying the potential thereof to effect the complete illumination of the said indicator, means for connecting said amplifier circuit and said balancing potentiometer in series with said high resistance medium, a buffer potentiometer in said series circuit, a variable resistance in circuit with said high resistance medium for compensating for the temperature of the said solution, a standard voltage cell, and means for connecting said cell in circuit with the balancing potentiometer for balancing the potentiometer against the standard cell so as to obtain a definite potential drop across the balancing potentiometer.

4. In an apparatus for indicating potentials, a high resistance medium permeable to hydrogenions and adapted to be interposed in contact with a solution to be tested, a balancing potentiometer including a variable resistance, said resistance being graduated in pH units for indicating the potential of said solution due to the hydrogen-ion concentration therein in terms of acidity of said solution, a direct current voltage amplifier circuit including a visual indicator and means in said circuit for varying the potential thereof to effect the complete illumination of the said indicator. means for connecting said amplifier circuit and said balancing potentiometer in series with said high resistance medium, a butler potentiometer, means for connecting said buffer potentiometer in said series circuit, said bufler potentiometer including a variable resistance adapted to be adjusted for balancing out the juncture potentials in said series circuit and to eflect the complete illumination of said indicator when the juncture potentials in said circuit are balanced out, and said graduated variable resistance being adapted to be adjusted to balance the same against the said high resistance medium and when in balance therewith to eflect the complete illumination of said indicator.

SAUL B. YOUNG. 

